The invention relates to a method and a receiver for receiving a spread spectrum signal in a radio system employing the code division multiple access method.
In mobile systems, information is transferred between a mobile network and a mobile station by means of radio resources. The radio resources of a mobile network are defined in different ways depending on the multiple access method of the system. Those using the frequency division multiple access (FDMA) method are distinguished from each other on the basis of the frequency used. In radio networks using the time division multiple access (TDMA) method, several users are able to communicate in the same frequency band, in which users are distinguished from each other in time by dividing the information transmitted or received into timeslots. In radio systems using the code division multiple access (CDMA) system, several transmitting and receiving stations communicate simultaneously in the same frequency band of the radio spectrum. A spreading code for spreading the information in a base band signal is reserved for each user for the duration of a connection. The receiver of the signal, in turn, is able to identify the information transmitted by the user by decoding is using a corresponding despreading code. Compared with other systems using multiple access methods, the advantages of CDMA include efficient utilization of a frequency band and system data security. A disadvantage is that users operating in the same frequency band interfere with each other""s transmissions because spreading codes and despreading codes are not orthogonal and transmitters are not mutually synchronized.
In addition to the interference caused by users to each other, for example the shapes of the surrounding terrain interfere with transmissions of information on the radio path. Multipath propagation means that, when propagating, a user signal is reflected from various points causing components from the same signal to arrive at a receiver at various delays. The multipath propagated signal components may cause fading, for example, in a situation when a signal is reflected from two adjacent objects. When the fading is significant, the reception of the signal fails altogether. This problem can be avoided for example by implementing the transmission on a radio channel by means of frequency hopping, whereby the transmission frequency is changed for each burst. In channel coding, the data to be transmitted are coded with a coding algorithm, allowing the authenticity of the coding and, consequently, of the received data to be verified at the receiving end. When transmitted interleaved, the data are divided into several bursts, whereby the loss of one burst is not fatal to the transmission of speech, for example. Furthermore, the receiver can be implemented with more than one antenna to gain antenna diversity, whereby the loss of a signal in one antenna is not fatal to the reception of the signal; instead, another antenna can receive the signal. The implementation of antenna diversity sets high requirements on the apparatus used, wherefore in practice it is preferably implemented in a base station of a mobile network.
In a RAKE type of CDMA receiver, multipath propagation may be utilized such that components that are delayed in various ways are received and combined to achieve optimal user signal identification. In a RAKE receiver, a delay profile, i.e. impulse response, is typically generated for a radio channel. A matched filter (MF), for example, can be used to generate the impulse response. A matched filter is used for example for information received on a pilot channel or in the pilot sequence of a radio burst. Pilot symbols are a group of symbols known to the receiver and transmitter, whereby the receiver of the information, being aware of what the received information should have contained, is able to make an estimate of the quality of the radio channel. A matched filter is shifted over the received information for example one half of a spreading code unit (chip) at a time, and the reception power is measured always when the matched filter synchronized with the received information. This allows an impulse response graph containing information on the signal strengths and delays of the multipath propagated components to be formed for the multipath propagated components of the received radio channel.
Let us assume that the measuring period of the impulse response is divided into N sequences of the length of L chips, the length of the measuring period being NLTc, wherein Tc is the duration of a chip. An impulse response estimate Pi(a)(t) is generated for each sequence i=0, . . . , Nxe2x88x921 of length L, and antenna 0, . . . , Na xe2x88x921 in accordance with formula (1):                                                         P              i                              (                a                )                                      ⁡                          (              t              )                                =                                    "LeftBracketingBar"                                                1                  L                                ⁢                                                      ∑                                          j                      =                      0                                                              L                      -                      1                                                        ⁢                                                                                    γ                                                  (                          a                          )                                                                    ⁡                                              (                                                                                                            (                                                              iL                                +                                j                                                            )                                                        ⁢                                                          T                              c                                                                                +                          t                                                )                                                              ⁢                                          c                                              iL                        +                        j                                            *                                                                                  "RightBracketingBar"                        α                          ,        wherein                            (        1        )            
xcex3(a)(t) is a signal received from antenna a, ci* is a spreading code and a is either 1, corresponding to an absolute value or 2, corresponding to squaring. The spreading code may be a real or complex value. The denotation ci* refers to the complex conjugate of a complex spreading code. The final power intensity for antenna a as a function of the propagation delay t is given as an incoherent mean over N measuring periods according to formula (2):                                           P                          (              a              )                                ⁡                      (            t            )                          =                              1            N                    ⁢                                    ∑                              i                =                0                                            N                -                1                                      ⁢                                                            P                  i                                      (                    a                    )                                                  ⁡                                  (                  t                  )                                            .                                                          (        2        )            
In a prior art receiver based on antenna diversity, each antenna branch has a matched filter, on the basis of which symbol estimates are separately and independently generated for a user signal irrespective of the other antenna branches. By combining the generated symbol estimates, better estimates for the symbols transmitted in the user signal are obtained than by generating the symbol estimates on the basis of a signal received in one antenna only. Said matched filter can be implemented for example time dividedly, allowing the matched filter""s processing capacity to be shared by several users. The processing capacity of a matched filter may also be divided time dividedly between antennas, allowing, in the case of for example two antennas, a user to be received during a first reception timeslot in a first antenna, antennas to be changed and the user received during the next reception time-slot in another antenna. Other users would be received in the following time-slots, until it is again said user""s turn, who is first received in antenna one and then in antenna two. The code phases of the RAKE branches are selected using the impulse response P(a)(t) measured for the antenna or the mutual impulse response of antennas directed to the same sector in accordance with formula (3).                                           P                          (              a              )                                ⁡                      (            t            )                          =                              1                          N              a                                ⁢                                    ∑                              a                =                0                                                              N                  a                                -                1                                      ⁢                                                            P                                      (                    a                    )                                                  ⁡                                  (                  t                  )                                            .                                                          (        3        )            
The prior art solution for the reception along several antennas in a receiver involves significant drawbacks. A reception solution where the antennas used for receiving a user signal are changed at given intervals is not well applicable to the reception of a radio channel containing fading. For example in the case of a time-divided matched filter, when antennas are changed for the user at given reception timeslots, the problem is that the reception time for each antenna becomes short. Accordingly, when hitting said short timeslot, a fading gap significantly disturbs the reception of the signal. In this case the averaging time of the impulse response becomes short, emphasizing the harmful effect of a fading gap in the generation of the impulse response. Furthermore, in a matched filter where reception antennas are changed at intervals of reception timeslots, a processing delay is caused during change of antennas when the matched filter synchronizes with the new antenna.
The object of the invention is thus to provide an improved method and apparatus for receiving a signal in a radio system. This is achieved by the method of receiving a spread spectrum signal in a cellular radio network to be described next. The method comprises receiving a spread spectrum signal containing several user signals in two or more antennas of a receiver in a cellular radio network, delaying the spread spectrum signal received in at least one antenna to prevent the spread spectrum signals received from the different antennas from being cancelled, combining the spread spectrum signal received in at least two antennas to form a combination signal, and generating a user signal combination impulse response by means of the combination signal.
The invention also relates to a RAKE receiver for receiving a spread spectrum signal. The RAKE receiver comprises at least two antennas for receiving a spread spectrum signal containing several user signals, at least one delay unit for delaying the spread spectrum signal received in at least one antenna to prevent the spread spectrum signals received from the different antennas from being cancelled, an adder for combining the spread spectrum signal received in at least two antennas to form a combination signal, and a matched filter for generating a user signal combination impulse response by means of the combination signal.
The invention relates to a method and receiver for receiving a spread spectrum signal in a radio system applying the code division multiple access method. The radio system is preferably a mobile telephone system, but the invention is, however, not restricted thereto. The receiver is preferably of the RAKE type, where a RAKE branch can be allocated to the reception of each multipath propagated signal component. The receiver of the invention is a receiver applying antenna diversity, i.e. there are at least two reception antennas. The receiver of the invention is preferably implemented in a base station of a mobile system, where the implementation of antenna diversity is most practical.
A spread spectrum signal contains user signals spread with several different user spreading codes. In the receiver of the invention, a spread spectrum signal is received in two or more antenna branches, the signals received from which are delayed, and different signals combined to form a combination signal. The combination signal is processed in the receiver in a matched filter wherein a radio channel impulse response is generated. The impulse response is processed further to locate the antenna branch that received the signal component. Of the signal components, those yielding the best power level are further allocated to be received by the finger branches of the RAKE receiver.
In accordance with the above, the signals received in the different antenna branches are delayed with respect to each other to prevent signal cancellation. Delaying the signals of the different antennas depends on time such that in an embodiment the delay of an antenna branch is increased between reception cycles. Herein, a reception cycle refers to part of the reception period of a matched filter, the reception period being the time from which the impulse response is generated. An upper limit is preferably set for the antenna delay, and the delay is not increased above said limit, and an upper limit under which the delay is not lowered. In this case, when the delay of a given antenna reaches the upper limit, for example, the sign of the direction of growth of the delay of that antenna branch is set to be negative, whereby the delay decreases until it reaches the lower limit.
The spread spectrum signals received in at least two branches and delayed in different ways are combined in a receiver to form a combination signal. One matched filter is used to generate a radio channel combination impulse response in the receiver from the combination signal. In this case the combination signal to be received with one matched filter includes a spread spectrum signal received from at least two antennas. The combination impulse response is preferably processed further by generating an antenna-specific impulse response for each antenna, enabling the location of the antenna that received the signal component in the combination impulse response. In order to distinguish between the antennas, said antenna delays have to be selected suitably to find the signal components of the desired antenna. Of the antenna impulse response signal components found, the best are allocated to be received by RAKE fingers. In this case, the RAKE fingers receive signal from the antenna from which the original spread spectrum signal was received. Thus the signal received by the RAKE fingers is an undelayed, not yet summed antenna signal.
The invention provides advantages. Because a spread spectrum signal received from two or more antenna branches is combined in the solution of the invention, one matched filter can simultaneously process a signal received from two or more antennas. This eliminates the processing delay associated with the change of antennas in the matched filter. Furthermore, the solution of the invention allows the averaging time of the calculation of the impulse response to be lengthened, resulting in a less probable loss of a signal in a fading channel than in using a short averaging time.